Plastic lenses are lighter, less in cracking and tintable in comparison with inorganic lenses. Therefore, in recent years, the application of plastic lenses to optical elements such as spectacle lenses and camera lenses has increased rapidly.
For this reason, the resin for plastic lenses having improved performance is desired. Further, the resin for plastic lenses having higher refractive index, higher Abbe number, lower specific gravity and higher heat resistance is desired. Until now, the resin material for various lenses has been developed.
Among them as to the representative examples, the polythiourethane resin having high refractive index by increasing a sulfur contents in thiol, which is used as polythiourethane, is developed (See Patent Documents 1 and 2). Among them, the polyurethane resin using alicyclic isocyanate compound represented by the formula (1) and/or the formula (2) (it will be described later) provides plastic lenses having incredibly excellent balance of high Abbe number, high heat resistance (See Patent Document 3).
However, although the above polythiourethane resin is provided with improved high Abbe number, heat resistance, as well as some extent of improved refractive index, higher refractive index is desired. Further, the polythiourethane resin using alicyclic isocyanate compound represented by the formula (1) and/or (2) and the polythiol represented by following formula:
gives high refractive index, Abbe number, and heat resistance (See Patent Document 4).
In the above polythiourethane resin, it has high refractive index, high Abbe number, and high heat resistance. However, if the resin has higher refractive index, the specific gravity tends to be higher and the strength of the resin is insufficient, and during processing of the resin, problem of occurring of cracking or breaking of the resin arises. In this regard, it has a problem that the yield is poor. Further, if heat resistance is lower, the crack is occurred on using after coating treatment. Further, if heat resistance is higher, resin strength tends to be poorer. For these reasons, the resins having a given refractive index, heat resistance, strength, and specific gravity in balanced manner are demanded.    Patent Document 1 Japanese Laid-Open Publication HEI2-270859    Patent Document 2 Japanese Laid-Open Publication HEI7-252207    Patent Document 3 Japanese Laid-Open Publication HEI3-124722    Patent Document 4 Japanese Laid-Open Publication HEI9-110956